1. Field of the Invention
The present invention pertains to solutions of silsesquioxane and siloxane resins. More particularly, the invention pertains to a solvent composition for forming improved stability coating compositions containing such resins.
2. Description of the Prior Art
It is known in the art that silsesquioxane and siloxane resins are useful in the electronic and semiconductor fields to coat silicon chips and other similar components. Such coatings protect the surface of substrates and form dielectric layers between electric conductors on integrated circuits. Such coatings can be used as protective coatings, interlevel dielectric layers, doped dielectric layers to produce transistor like devices, pigment loaded binder systems containing silicon to produce capacitor and capacitor like devices, multilayer devices, 3-D devices, silicon on insulator devices, coatings for superconductors, superlattice devices and the like. These resins include hydrogen silsesquioxane resins as well as silsesquioxane resins containing a significant portion of organic moieties.
The production of silsesquioxane resins is well known in the art. U.S. Pat. No. 3,615,272 teaches the production of a nearly fully condensed resin which may contain up to 100-300 ppm silanol by a process comprising hydrolyzing trichlorosilane in a benzenesulfonic acid hydrate hydrolysis medium and then washing the resultant resin with water or aqueous sulfuric acid. U.S. Pat. No. 5,010,159, teaches a method comprising hydrolyzing hydridosilanes in an arylsulfonic acid hydrate hydrolysis medium to form a resin which is then contacted with a neutralizing agent. Other hydridosiloxane resins, such as those disclosed in U.S. Pat. No. 4,999,397 are produced by hydrolyzing an alkoxy or acyloxy silane in an acidic, alcoholic hydrolysis medium. The use of hydrogen silsesquioxane resin in forming coatings on electronic devices is also well known in the art.
In order to effectively coat the resins onto a substrate, they are dissolved in a solvent to form coating compositions. Organic solvents are frequently used for this purpose. However, silsesquioxane resins (e.g. hydrogensilsesquioxane resin) dissolved in organic resins tend to have relatively short shelf lives. That is, the resins tend to undesirably increase in molecular weight during storage in a relatively short period of time. To date, it has been found that methylisobutyl ketone (MIBK) provides the longest shelf life of the commercially available solvents and therefore MIBK is widely used in the industry. Unfortunately, MIBK presents environmental and handling concerns and the industry has sought to reduce MIBK use. Prior to this invention, no successful alternatives to MIBK have been found. Other prior art solvents form coating composition with silsesquioxane resins which are unstable. U.S. Pat. No. 4,756,977 teaches a process of forming a coating on an electronic device comprising diluting hydrogen silsesquioxane resin in a solvent, applying the solution to the electronic device and heating the coated device to convert the resin to a ceramic. This patent lists toluene and n-heptane as exemplary solvents. Similarly, U.S. Pat. No. 5,320,868 teaches solvents which can be used for coating hydrogen silsesquioxane resin including alcohols such as ethyl or isopropyl, aromatic hydrocarbons such as benzene or toluene, alkanes such as n-heptane or dodecane, ketones, cyclic dimethylpolysiloxanes, esters or glycol ethers. This patent does not mention the solvents disclosed herein. U.S. Pat. No. 5,370,904 teaches solvents including MIBK and certain silicones which can be used for coating hydrogen silsesquioxane resin, however, there is no appreciation of the improvement in stability when silicone solvents are used as opposed to non-silicone solvents.
It would be desirable to produce improved coating compositions with silsesquioxane resins which are storage stable. It has now been found that stable solutions can be formed to deposit coatings which still have excellent coating quality. The solvent composition of this invention for coating silsesquioxane and siloxane resins provides shelf life properties that are equal to or better than MIBK and which is safer and more environmentally acceptable than MIBK. It has now been found that when silsesquioxane and siloxane resins are dissolved in solvents containing certain siloxane functional groups, the compositions exhibit good coating properties, have very good shelf life, and present fewer handling and environmental concerns than when MIBK is used in the solvent.